Rafael Lozano-Hemmer is turning the Park Avenue tunnel in New York into a reverberating sound and light installation that celebrates free speech. From 3 to 17 August, this tunnel will be closed to traffic and open to visitors for Voice Tunnel, part of New York's Summer Streets festival. You walk into the middle of the tunnel, where a silver intercom awaits your remarks. Speak a short message and it will pulse through the tunnel in waves of sound and light.

If you've ever sat in a cinema in Manhattan and enjoyed the continual stream of jokes and comments with which New Yorkers give a film instant feedback, it's easy to imagine some salty, witty and creative words shaking the Park Avenue tunnel. In fact, the New York Police Department had serious anxieties about what kind of words might be spoken. The NYPD asked the artist to install a time delay so messages could be regulated. Lozano-Hemmer refused, telling the New York Times: "This is the place for people to express their views. That's what this project is about. And if you want to censor it – I've never in my life censored a work, and I won't do it."

I'm sure the free speech that echoes through the tunnel will be vivid, but it is unlikely anything the police department will need to fret about. For one thing, New Yorkers seem to say what they want where they want – it's unlikely anyone will walk into the centre of a tunnel just to say the unsayable. And the art audience that gathers will likely create its own civil context.

Ideally, free speech is supposed to regulate itself. In a healthy public space, people can argue, debate, joke and even criticise each other without the air turning thick with threat. Call this politeness, call it civilisation, call it a necessary veneer of hypocrisy – we need it and when it breaks down, things get ugly.

The other night, I took a train from Edinburgh to London. At York, a group of noisy, boozed-up people got on and sat behind me. After a while a man pointed out this was the "Quiet Coach". The York group responded furiously to his complaint. A woman supported him, again stressing that they were in the Quiet Coach. What happened next was hideous.

Several men in the party of drinking companions rounded viciously on the woman, insulting her, deconstructing what they imagined to be her personality and, in short, subjecting her to sustained misogynist remarks.

Such verbal attacks do not only happen online – they can happen face-to-face in a railway carriage. After telling these men what I thought of them, I found the station guard, who had to threaten to involve the police just to modulate the tone of the "conversation".

This explosion revealed how free speech can be noxious, after all.

Raphael Lozano-Hemmer is right to glorify the human voice. But without civility the loudest and most aggressive voices win.